1. Field of the Invention
This invention relates to a negative electrode for a non-aqueous electrolyte secondary battery which uses an organic electrolyte with lithium salt dissolved in an organic solvent.
2. Description of the Prior Art
Reflecting the recent trend of increasing demands for smaller and lighter power supplies for use in portable electronic appliances, research and development activities on non-aqueous electrolyte secondary batteries with lithium (Li) as the negative active material are taking place actively in many countries of the world. These batteries usually have a higher operating voltage, requiring a high energy density. The well-known positive active materials for such batteries include chalcogenides of oxides or sulfides of transition metal such as MnO.sub.2, V.sub.2 O.sub.5, Cr.sub.3 O.sub.8, TiS.sub.2, MoS.sub.2, etc. and also such double oxides as LiMnO.sub.2, LiMn.sub.2 O.sub.4, LiCoO.sub.2, LiNiO.sub.2 and the like. These active materials are of either layer or tunnel construction and have a crystal structure wherein Li ions are absorbed by discharge and desorbed by charge. On the other hand, the pure metal Li shows the highest operating voltage per unit cell as the negative electrode and studies have been conducted on Li for many years. However, the reaction wherein the metal Li is deposited by charge and dissolved by discharge is not necessarily reversible. During the times of discharge, dendrite or mossy bulk of Li crystal is deposited on the negative electrode surface, penetrating through separators and causing an internal short circuiting with a resultant shorter charge and discharge cycle life. Moreover, a shape change of the negative electrode and deterioration in the charge and discharge efficiency that develop with charge and discharge cycles will result in the problem of a capacity reduction of the negative electrode or the cell. As a solution, for example, to the problems of dendrite crystal deposition on the negative electrode and also a shape change of the electrode, metals like Al, Al alloy, Wood's metal and the like which easily alloy with Li are used as the holder of the active material. In this case, the metal Li deposited by charge is immediately diffused to make a Li alloy. During the times of discharge as the Li in the Li alloy formed on the negative electrode surface dissolves into the electrolyte, the Li of the Li alloy in the negative electrode moves by diffusion towards the negative electrode surface. Also, in this case a deep charge and discharge cycle application will cause a particle size reduction of the metal powder that forms the Li alloy and the active material holder, resulting in a great increase of the volume and the specific surface area of the metal powder and eventually a shape change of the negative electrode. In the case of a strip type negative electrode that uses a foil collector as the electrode core, defects of exfoliation between the foil collector and the metal powder layer of the active material holder will develop. Besides, a more active negative electrode surface due to increased specific areas of the metal powder tends to bring about such problems as ignition and the like.
In this case, such materials as carbon like graphite, sulfide, oxide or conductive polymer, wherein Li ions are doped by charge to form a Li compound and Li ions are undoped by discharge, may be used as the holder for the active material. As far as overcharging is not taking place, this arrangement is safe and makes a relatively high charge rate possible. Especially, the use of carbon material as in the foregoing has been receiving a special attention in recent years with the resultant established technology of so called "lithium ion rechargeable battery".
The non-aqueous electrolyte secondary battery using metal, carbon, oxide, sulfide, etc. in place of Li itself as the holder for the negative active material as described in the foregoing was produced in the beginning with a configuration of coin type or button type cell for the main application of memory back-up. However, as a light-weight compact main power source for the portable equipment, a cylindrical cell was developed and is actually used. This is constructed by having one sheet each of thin and long strip type positive plate and negative plate wound spirally with a micro-porous polypropyrene separator inserted in between, forming an electrode group. The electrode group is placed in a cell case to complete a cylindrical cell. The foregoing electrode group is constructed in such a way that organic electrolyte is fixed by absorption and no free electrolyte is existent. In this way, the facing area between positive and negative plates is increased, the electrode spacing is made uniform and free electrolyte is eliminated, consequently contributing to a uniform charge and discharge reaction due to a low current density and also to suppressed generation of dendrite crystal Li. The thin and long strip type positive and negative plates are made by coating the active material itself containing some binder or the active material holder, mixed with a necessary conductive material and formed to a paste-like condition, onto a collector of metal foils and then by drying and pressing the coating.
Since the electrode plates are to be wound, they have to be flexible and their surface needs to be smooth and also selection of the binder is very important. When metal foils are used as the collector, they have to be non perforated ones for the extension strength needed to build the electrode plates and for the flatness and the smoothness of the electrode plate surface. Therefore, the binder as used with the negative electrode for non-aqueous electrolyte secondary batteries that absorbs Li by charge and desorbs it by discharge and comprises powdered metal, carbon, sulfide, oxide and the like must have the following characteristics:
(1) It is not dissolved by the organic electrolyte used together. PA1 (2) It is not affected by reduction that takes place at around the electrode potential of Li. PA1 (3) It has a strong binding strength that holds together the particles of the powdered active material holder. PA1 (4) It provides a strong binding strength working between the non-perforated metal foils and the active material holder, not causing any exfoliation between the two. PA1 (5) Not only is its binding strength strong but its flexibility is sufficient.
So far, as the binder for the negative active material holder, polytetrafluoroethylene (PTFE) and the polyolefin group resins such as polyethylene (PE) that are usually used with the positive electrode have been studied. PTFE assumedly reacts with Li and tends to lower negative electrode capacity. In the case of PE, due to expanding and shrinking of the active material holder that accompanies with absorption and desorption of Li, the bonding between particles is not strong enough and especially exfoliation between the foil collector and the layer of active material holder tends to occur easily.